There is not much point in considering military aerodynamic configuration development without including stealth. It plays a key role in the configuration layout. A few government planners want to ignore the fundamental importance of stealth to survivability. This is fanciful and nostalgic thinking. The fact is that missiles are finally becoming reliable, and there is no such thing as too much stealth. Although the details are classified, certain basic principles have been described.

Stealth is usually considered to consist of several elements (often referred to as signatures):

• radar cross section (RCS)
• infrared
• visual
• aural

For aerodynamic configuration design, the key element is radar cross section, RCS, with some consideration of infrared, mainly from the back of the aircraft. In the mid-1980s, I actually took the graduate sequence in electromagnetic theory at a local university. Any aerodynamicist working in military configuration design will have to add this topic to his plan for continuing education.

Although complete information on this technology is not available, there are numerous references that define the public’s knowledge of stealth. To get insight into how stealth emerged to influence airplane design, read the book by Ben Rich.^[1] This book tells the story of stealth configuration development at the Skunk Works. Not particularly technical, but a good read, providing insight into aircraft development programs. Rich succeeded Kelly Johnson, the founder of the Skunk Works. Rich headed the Lockheed Skunk Works during the development of the F-117 but passed away much too soon. More details on the F-117 design are given by Alan Brown.^[2] The B-2 development is described in part of the 1991 Wright Brothers Lecture by Waaland.^[3] The paper describes experiences at Northrop, including the B-2 development, along with many other programs. The author received the AIAA Aircraft Design Award for the B-2.

Explicit discussions of stealth in airplane design have been given by Raymer.^[4] The section on RCS is found on pages 191 to 201 of Raymer’s book. Infrared, visual and aural aspects of stealth are discussed in the following sections on pages 201 to 203. This is a good direct unclassified source of accurate shaping information. A very good SAE paper by Whitford^[5] was adapted by the author in “Fundamentals of Fighter Design, Part 10 - Stealth” (in Sept. 1997’s Air International) and his book, Fundamentals of Fighter Design (published by Airlife in 2000). Somewhat more theoretical treatments of the theory underlying stealth have been given by Ball.^[6] More insight into electromagnetic theory can be obtained from the handwritten charts from lectures by Prof. Fuhs.^[7] These were developed in 1982, but there are no date or copyright markings. The synopsis given here is supported by these references. A good overview of the survivability issues has been given by Patterson,^[8] who discusses the question of how much stealth is enough.

B.1 How RCS Works

(1) A radar site transmits a signal and measures the signal that is returned from the target (in this case, an airplane).

When the sending and receiving antennas are colocated, the radar is known as monostatic, as shown in figure B-1(a). This is the usual case. If the receiving antenna is located somewhere else, the radar is bistatic, shown in figure B-1(b). Bistatic systems may be able to detect aircraft designed to operate stealthily against monostatic systems. This is a fundamental consideration in stealth.

(2) There is a length scale associated with radar:

$$\underset{\text{wavelength}}{\lambda }\cdot \underset{\text{frequency}}{\underbrace{f}}=\underset{\begin{array}{c}\text{speed of}\\ \text{light}\end{array}}{c}\text{(B-1)}$$

The ratio of the wavelength to key length scales on the vehicle,

$$\frac{\lambda }{l_{ref}}\text{,}\text{(B-2)}$$

is important in understanding the physics of the radar reflectivity. Several different mechanisms exist, and these ratios can be thought of (very) loosely as analogous to the Reynolds number and Knudson number for use in aerodynamics, where values of these parameters are used to decide which physical phenomena dominate the flow field. The wavelength also determines the size of the antenna required.

(3) The signature is expressed as an area.

One square meter is the reference area, and the value of the RCS is usually expressed as a relative value using decibels:

$$\sigma \left(d{b}_{sm}\right)=10\cdot {\log }_{10}\left(\frac{{\sigma }_{\text{meters}}}{1\text{meter}}\right)\text{(B-3)}$$

Ben Rich loved to tell the story of his test range experience, where the operator claimed that the model, a precursor of the F-117, wasn’t “on the pole” until a bird landed on the model and he could pick up a reflection. That should tell you something about the signature level of the F-117.

For a lot of the work in aerodynamic configurations, specular reflection dominates, making physical optics useful. Figure B-3 is a sketch based on Fuhs’s notes that illustrates the situation. It is perhaps obvious, but to avoid large radar returns, there should be no surfaces normal to the radar signal.

Clearly, flat surfaces normal to the incoming waves are bad and reflect strongly back to the transmitter, thus surfaces should be angled to reflect the waves in other directions, as illustrated in figure B-4.

Designers work to different RCS target values (levels) in different sectors. A typical division is shown here in figure B-5.

The front sector typically has the lowest allowable RCS value. This means that wings are swept and cavities are bad. The worst case is the inlet and engine front face.

The F-14 and F-15 aircraft turned out to have terrible inlets from a stealth point of view. This was ironic. The designers had worked hard to design these intakes since they were excellent aerodynamically. Figure B-6 illustrates this situation.

Instead of the F-15 type inlets, the engine front face has to be shielded by an offset inlet, as shown below in figure B-7. Observe the extreme effort devoted to hiding the engine in the F-117 and B-2. This also provides an opportunity to take full advantage of radar absorbing material (RAM) treatments in the duct. Thus modern military inlets use S-shaped inlets; figure B-7 provides an example. More information and new approaches to inlet design appeared in Aviation Week.^[9]

Cockpits and radomes are also bad, passing electromagnetic waves to the surfaces inside them, which are often huge reflectors. Thus special design procedures and materials are required to reduce the radar cross section.

From the side, vertical surfaces are eliminated, introducing canted tails and chine-sided fuselages. However, corner reflectors are terrible, so the angle shown in the front view is only acceptable if it doesn’t line up in the side view. Figure B-8(a) illustrates the poor shaping situation where the configuration is prone to reflecting lots of incident waves.

Figure B-8(b) is the good shaping situation since incident waves will be scattered in different directions and only a small portion will be returned in the direction of the incident waves. Note that care must be taken to ensure that the vertical tails do not form a corner in the side view since this is even worse than the arrangement shown in figure B-8(b).

This also explains the sawtooth landing gear doors and access panels illustrated in figure B-9.

Finally, in addition to shaping, the vehicles are treated with coatings and special materials to reduce the radar return.

B.2 Computations

The second edition of Ball’s book on aircraft combat survivability^[10] provides some sources to start making RCS estimates. Sefer et al.^[11] describe MATLAB-based methods for RCS analysis. An article by Joy and Singh^[12] provides hybrid approaches incorporating low-frequency and high-frequency methods to solve electromagnetic problems.